Experimental set-up for the study of dynamic self-assembly of spinning disks. A bar magnet rotates at angular velocity q below a dish filled with liquid (typically ethylene glycol/water or glycerine/water solutions). Magnetically doped disks are placed on the liquid-air interface, and are fully immersed in the liquid except for their top surface. The disks spin at angular velocity q around their axes. A magnetic force Fm attracts the disks towards the centre of the dish, and a hydrodynamic force Fh pushes them apart from each other.

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Figure 2.

Dynamic patterns formed by various numbers (n) of disks rotating at the ethylene glycol/water-air interface. This interface is 27mm above the plane of the external magnet. The disks are composed of a section of polyethylene tube (white) of outer diameter 1.27 mm, filled with poly(dimethylsiloxane), PDMS, doped with 25 wt% of magnetite (black centre). All disks spin around their centres at q = 700 r.p.m., and the entire aggregate slowly (O < 2 r.p.m.) precesses around its centre. For n

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Figure 3.

Dynamic self-assembly of bubbles generated by multi-step flow focusing device. A single breakup event generated various combinations of multiple bubbles (artificially colored black using Photoshop for easier interpretation), ranging from bi-disperse bubbles (Fig. 1a – the larger bubble from one breakup associated with the smaller bubble from the previous break-up) to tri-disperse bubbles (Fig. 1b - bubbles of three different sizes from a single breakup associated together as the two smaller bubbles flowed around opposite sides of the largest bubble).

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Figure 4.

Optical micrographs showing the formation of droplets of water in hexadecane in coupled flow-focusing generators. The mutual interaction between droplets generated by multiple flow-focusing devices resulted in the in-phase mode of operation of the generators (marked with solid rectangles) or the out-of-phase mode of operation of the generators (marked with dashed rectangles). Water droplets were stabilized with surfactants (Span 80, 3% w/w).

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Figure 5.

A model system for examining eruptions in spreading fires: an ignited strip of nitrocellulose rests on a suspended wire mesh. A bump in the strip can, with some probability, transition small, slowly moving flames into intense, rapidly moving flames (left column), or vice versa (right column).

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Whitesides Group Research

Dissipative Systems

Many natural processes (e.g., fires, life) operate out of—and, often, far from—thermodynamic equilibrium. Often, these dissipative processes, and the systems in which they occur, spontaneously become more “complicated” while dissipating energy. That is, they develop patterns, structures, or behaviors that they did not have when first formed. We are interested in the self-organization of dissipative systems; we aim to create simple model systems that enable us to study the origin, structure, and stability of this organization.

Rotating objects at a liquid-gas interface

We studied the organization of spinning disks located at the interface between liquid and air (1). We drove the rotation of disks magnetically: each disk was filled with ferromagnetic powder, and we drove their rotation by spinning a permanent magnet under the dish that contained the disks (Figure 1). A single disk drifted to the axis of rotation of the magnet; multiple disks interacted repulsively and self-organized into ordered structures (Figure 2). As the disks rotated around their axis, they engaged the liquid near them and the resulting fluid flow generated hydrodynamic lift forces that led to an effective repulsion between the disks. The dissipation in this system was due the viscous flow and was critical to the formation of repulsive forces that stabilized the patterns of disks. Viscous friction enabled the engagement of the fluid near the disks, and ensured that the flow of fluid in the dish was laminar with a finite Reynolds number – a condition under which hydrodynamic lift forces led to the repulsion between disks.

Nonequilibrium structures in multiphase microfluidic flow

We are exploring the formation of bubbles in a microfluidic flow-focusing device (Figure 3) in which the rate of flow of liquid and the pressure of gas are externally controllable (2). Over much of the flow rate/pressure phase space, the system produces monodisperse bubbles. We have shown that these bubbles can be used to generate flowing lattices and dynamically assembled foams (Figure 4). As one of the parameters is varied, however, the sizes of the bubbles produced become bi-disperse (Figure 4). Further variation of the parameter leads to periodic production of bubbles of four different sizes. The flow-focusing device can also be tuned to produce bubbles with a random size distribution. The system shows similar behavior to a dripping faucet, which also displays period-doubling bifurcations.

Spreading flames

Our work in the area of flames has focused on (i) multistability and critical transitions in complex systems and (ii) the spread of signals through interconnected networks. Recently, we have constructed a model system to examine the eruption of small flames into intense, rapidly moving flames stabilized by feedback between wind and fire (i.e. “wind-fire coupling”—a mechanism of feedback particularly relevant to forest fires) (3). Using this model, we showed that slowly spreading flames can exhibit detectable symptoms of critical slowing down (i.e. the slowed recovery of multistable systems from perturbations as those systems approach tipping points) prior to such eruptions. This finding, which marks the first demonstration of critical slowing down in a combustion system of any kind, suggests that slowing responses of spreading flames to sudden changes in environment (e.g. wind, terrain, temperature) may anticipate the onset of intense, feedback-stabilized modes of propagation (e.g. “blowup” events in forest fires).